Gasification of solid fuel such as coal, biomass or various wastes in a gasification furnace is generally carried out in a high-temperatured environment of about 1123 K or more so as to obtain sufficient reaction speed and heat supply to the reaction. In order to attain such high-temperatured environment of about 1123 K or more in the gasification furnace, part of the solid fuel itself must be burned.
However, such combustion of the solid fuel itself disadvantageously deteriorates gasification efficiency of the fuel. To carry out the combustion and gasification of the fuel in one and the same reaction space or gasification furnace inevitably causes a large amount of inert gases such as CO2 and N2 to be admixed in the gasified gas, resulting in lowering in purity and heat quantity of the product gas.
Moreover, the gas gasified in the high-temperatured environment is rich in CO and CO2 and poor in H2; in order to produce H2-enriched product gas required, for example, for a synthesizing process of GTL (Gas to Liquid), the high-temperatured gasified gas must be cooled to independently carry out CO shift reaction and removal of CO2.
A conventionally known method for concurrently removing CO during gasification of solid fuel is to absorb CO2 in gasified gas in a gasification furnace, using a chemical such as CaO-based oxide; however, in a high-temperatured environment of 1123 K or more, there is a restriction in terms of chemical equilibrium that absorption of CO2 requires the gasification furnace to be in a high pressure environment of 20 atm or more (see, for example, Patent References 1 and 2).
The gasification technique at such high pressure can be utilized practically only in large-scaled energy/fuel producing systems of several hundreds MW from a viewpoint of cost or other restrictions; in other various low-capacity systems such as a dispersed hydrogen fuel cell power and synthesis system, it has been desired that production of H2-enriched product gas be carried out through gasification at low or preferably normal pressure.
Thus, it is conceivable that a gasification process with enabled high efficiency at low or medium temperature and at low pressure is indispensable for application to various energy-scale energy/fuel production systems including the above-mentioned GTL or in order to construct next-generation, highly effective electric generating systems.
More specifically, if gasification at low or medium temperature were put into practice, there would be no need of burning the solid fuel itself; instead, for example, various industrial waste heats such as heat of exhaust gas from a gas turbine may be utilized as heat source for gasification with expectation for high efficiency of the gasification. There would be no need of a high pressure environment; instead, for example even at a normal pressure, CO2 in the gasified gas may be satisfactorily absorbed by oxide chemical such as CaO, providing that it is at low or medium temperature.
With respect to a method for gasifying fuel through combustion of solid fuel itself (usual partial oxidation method, other method of not using a gasifying agent or auto-thermal gasification method, or other method of using a gasifying agent such as steam or CO2), a twin-circulating-fluidized-bed-type gasification technique is known (see, for example, Patent References 3 and 4) wherein inert gas such as CO2 generated by combustion and N2 fed through supply of air for combustion are prevented from being admixed in the gasified gas in such a manner that the solid fuel is gasified in the gasification furnace, the gasified char being burned in a combustion furnace separate from the gasification furnace, heat fluid medium being circulated between these gasification and combustion furnaces to transfer heat from the combustion furnace to the gasification furnace.
In such a gasification method with fuel gasification separate from char combustion and in order to absorb CO2 in gasified gas to produce H2-enriched product gas, there has been developed, in Europe, a gasification method called AER (Absorption Enhanced Reforming) wherein heat medium circulated between the combustion and gasification furnaces is added with CaO chemical (see Non-patent Reference 1). In the AER method, circulating fluidized bed is used; biomass is gasified in a gasification furnace adjacent to a downcomer in an environment of 873-973 K and at normal pressure, CO2 being absorbed by CaO chemical to obtain gasified gas with high H2 content and to accelerate the gasifying reaction, CaCO3 thus generated being regenerated into CaO in a riser combustion furnace and being circulated to the gasification furnace together with the fluid heat medium.    [Patent Reference 1] U.S. Pat. No. 4,231,760    [Patent Reference 2] JP2004-59816A    [Patent Reference 3] U.S. Pat. No. 4,568,362    [Patent Reference 4] AT405937B    [Non-Patent Reference 1] http://www.aer-gas.de